Silica-xerogels are partially hydrolyzed oxides of silicium. Hydrolyzed oxide gels can be produced by a sol-gel process, which has been used for producing ceramic and glass materials for many years.
The sol-gel process is based on hydrolyzation of a metal-alkoxide and subsequent polymerization of the metal hydroxides as follows:Si(OR)4+H2O→HO—Si(OR)3+ROH  1)HO—Si(OR)3+3H2O+ROH→Si(OH)4+4ROH  2)Si(OH)4+Si(OH)4→(HO)3Si—O—Si(OH)3+H2O  3)
When the polymerization reaction goes further, additional chains, rings, and three dimensional networks are formed, and a gel comprising water, the alcohol of the alkoxy group and the gel itself is formed. The sol may also contain other additives such as acids or bases used for catalysis of the reaction. If alcohol and water are now extracted from the gel by washing and evaporating, a xerogel is obtained.
During drying large shrinking occurs creating internal stresses into the gel. If the monolithic gel is not allowed sufficiently time to relax its internal stresses, it will crack. During drying further polymerization of the remaining OH-groups occurs. The continuing polymerization carries on for a long time after gelation. This is called aging. The further the polymerization goes on, the more stable the gel or xerogel becomes. However, at room temperature the polymerization will effectively stop after a few weeks aging and the xerogel will not become totally inert. If the temperature is raised, the polymerization reaction can be accelerated, further stabilization and shrinkage occurs, and more internal stresses are introduced into the xerogel.
If the temperature is raised high enough (around 1000° C. for monolithic Si-gels) the gel or xerogel becomes a pure oxide and there are no OH-groups present in the material. However, in case of pure oxides, the reaction rate is extremely slow. If the oxides are incorporated with other ions, such as Na, K, Mg, or Ca, the reaction rate can be greatly increased. The so called bioactive glasses are developed from these systems. The dissolution rate of these glasses is controlled by the composition and surface area of the glass. These glasses are melted above 1000° C.
The general principles of mixing organic substances with gels are well known. The basic idea is that an organic substance is added to the sol-stage of the sol-gel process. Then, after gelation, the organic part has become an inherent part of the material. In conventional glass melting processes, this is not possible at all because the temperatures are much too high for organic substances to survive.
The sintering temperature is naturally a limiting factor also for many substances in organically modified silicates (ORMOSILS). In the case of medicines, the sintering temperature is limited by the breakdown of the structure or functionality of the medicine. For proteins, enzymes, antibodies and whole cells, the sintering limit is as low as 40° C. since they will begin coagulating at and above that temperature.
Organic substances are generally added to silica gels to modify the natural properties of the silicates with those of the organic substances. Some combinations of dopants and matrices used thusfar are disclosed in Chemistry of Materials (1994) 6:1605-1614 (D. Avnir et al.).
Silicium sol-gel material directed for oral short term (less than 24 hours) drug delivery and methods of mixing drugs with silica-viscous sol have been described in Drug Development and Industrial Pharmacy (1983) 9 (1&2):69-91 (K. Unger et.al). The article describes a polycondensation in solution method, which starts with mixing polyethoxysiloxane (PES) with a solution of the drug in an appropriate solvent, giving a molecular scale entrapment of the drug in the polymer. The release rate of the drug is controlled by diffusion through the pores of the matrix material.
Published application EP 0680753 describes a sol-gel produced silica coating and particles containing a biologically active substance where the release rate of the active agent is controlled by addition of penetration agents, such as polyethylene glycol or sorbitol.
Published application WO 96/03117 discusses bone bioactive controlled release carriers comprising silica-based glass providing for the controlled release of biologically active molecules, their methods of preparation and methods of use. These carriers are stated to be prepared using a sol-gel-derived process.